Disc devices such as magnetic disc devices, optical disc devices, data array devices are generally provided with a cache memory in terms of system performance and the like. The cache memory is typically configured of a nonvolatile memory such as SRAM (Static Random Access Memory) and DRAM (Dynamic Random Access Memory) and stores data accessed during operation of the disc device therein. At next access request, when requested data exists in the cache memory, the data in the high-speed cache memory can be used without having to access a low-speed disc. Thereby, a response performance to the access request can be improved.
To further improve the response performance, in some disc devices provided with the cache memory, a part of HDD (Hard Disk Drive) is replaced with SSD (Solid State Drive) is known (for example, refer to Patent document 1). The SSD is a disc drive using a nonvolatile flash memory as a storage element. Although the SSD can read/write data at higher speed than the HDD, the SSD has a limitation in the number of times of erasure/writing from/to blocks. For this reason, according to a technique described in Patent document 1, by recording data in a section of a storage area, which is often rewritten (Write block), into the HDD and recording data in a section of the storage area, which is not rewritten so much, but is often read (Read block), into the SSD, the life of the SSD is extended while improving the response performance.    [Patent document 1] Unexamined Patent Publication No. 2009-163647
According to the technique described in Patent document 1, the life of the SSD can be extended while improving the response performance. However, disadvantageously, the technique described in Patent document 1 can be applied to only disc devices in which the storage area is divided into the Write block that is often rewritten and the Read block that is not rewritten and often read.
By the way, a technique of stratifying the cache memory into a primary cache, secondary cache, . . . is known as one of techniques for improving the response performance. This technique can improve the response performance even in disc devices in which the storage area cannot be divided into the Write block and the Read block. Because the response performance can be improved as the capacity of the secondary cache is larger, a mass storage SSD may be used as the secondary cache.
However, since the SSD has the limitation in the number of times of erasure/writing from/to blocks, when the SSD is used as the secondary cache, the following problem occurs. If an access request to a block that does not exist in the primary cache is made in the case where all cache pages (cache lines) of the primary cache are used, one block is expelled from the primary cache and is written to the secondary cache. Accordingly, when the SSD is used as the secondary cache, the writing processing of the block expelled from the primary cache disadvantageously shortens the life of the secondary cache.